Especially in the area of microlithography, apart from the use of components having a high precision, it is desirable to keep the position and the geometry of the components of the imaging device, e.g. the optical elements such as lenses, mirrors and gratings, unchanged during operation to the highest possible extent in order to achieve a correspondingly high imaging quality. The demanding desired properties with respect to accuracy (lying in the magnitude of a few nanometers or below) are nonetheless a consequence of the permanent desire to reduce the resolution of the optical systems used in fabricating microelectronic circuitry in order to push forward miniaturization of the microelectronic circuitry to be produced.
In order to achieve an increased resolution either the wavelength of light used may be reduced as it is the case with systems working in the extreme UV (EUV) range at working wavelengths in the area of 5 nm to 20 nm (typically at about 13 nm) or the numerical aperture of the projection system used may be increased. One possibility to remarkably increase the numerical aperture above the value 1 is realized in so-called immersion systems, wherein an immersion medium having a refractive index larger than 1 is typically placed between the last optical element of the projection system and the substrate to be exposed. A further increase in the numerical aperture is possible with optical elements having a particularly high refractive index.
It will be appreciated that, in a so-called single immersion system, the immersion element (i.e. the optical element at least in part contacting the immersion medium in the immersed state) typically is the last optical element located closest to the substrate to be exposed. Here, the immersion medium typically contacts this last optical element and the substrate. In a so-called double immersion system, the immersion element does not necessarily have to be the last optical element, i.e. the optical element located closest to the substrate. In such double or multiple immersion systems, and immersion element may also be separated from the substrate by one or more further optical elements. In this case, the immersion medium the immersion element is at least partly immersed in may be placed, for example, between two optical elements of the optical system.
With the reduction of the working wavelength as well as with the increase of the numerical aperture not only the desired properties with respect to the positioning accuracy and the dimensional accuracy of the optical elements used become more strict throughout the entire operation. Of course, the desired properties with respect to the minimization of imaging errors of the entire optical arrangement increase as well.
The deformations of the respective optical element and the imaging errors resulting therefrom are of special importance in this context. More specifically, it has turned out that evaporation effects of the immersion liquid contacting the optical element may introduce a considerable thermal disturbance into the optical element leading to relatively high local temperature gradients within the optical element. These high local temperature gradients resulting in considerable stresses introduced into the optical element which in turn will lead to increased imaging errors.
These evaporation effects are especially undesired at (ideally) dry areas of the immersion element which, under ideal conditions, should not be wetted by the immersion medium. However, since under real operating conditions the substrate to be exposed at certain points in time has to execute comparatively fast relative movements with respect to the immersion element, kinetic energy is transferred to the immersion medium leading to a certain sloshing movement of the immersion bath. This sloshing movement leads to an inadvertent wetting of these dry areas with parts of the immersion medium such as thin immersion liquid films or immersion liquid splashes etc. These typically randomly distributed and hardly predictable films or splashes are prone to easily evaporate leading to the undesired result on the imaging errors as outlined above.
To solve this problem it has been proposed to provide hydrophobic coatings at these dry areas of the immersion element to reduce the amount of immersion liquid which may contact the immersion element sufficiently long to evaporate and, thus, introduce a noticeable thermal disturbance into the immersion element. However, despite their hydrophobic properties, the use of such coatings has to rely on the gravitational force acting on the immersion liquid splashes or films to provide rapid removal of the immersion medium from the dry areas. Thus, particularly under unfavorable geometric conditions, these hydrophobic coatings may not be sufficient to provide rapid removal of the immersion medium prior to noticeable evaporation.